1. Field of the Invention
The present invention relates to an addressing system for an information processing apparatus or a microprocessor and, more particularly, to a bit addressing system which executes accessing in a bit unit.
2. Description of the Prior Art
In the processing of a data by means of a computer, it is general to make a prearrangement as to where the data is to be stored in what form. The method which indicates where the data is located is termed an addressing method while the prearrangement which tells in what form it exists is termed a data type. A data is addressed and the contents of the data are interpreted, processed, and transferred in accordance with instructions described in confirmity with such a prearrangement.
The minimum addressing unit for a memory is normally one byte (8 bits). That is, a memory is defined as a simple string of bytes, and the data is accessed through its byte address. A byte address is a value which uniquely specifies the position of the byte.
An instruction format consists generally of an array of an instruction code which shows an operation, and in addressing operand which specifies the operation data that become the objects of the instructed operation. The addressing operand includes a plurality of field to provide various addressing modes in accordance with the combination thereof. However, in the respectige addressing modes, the minimum addressing unit for a memory is still one byte.
On the other hand, there is an instruction requiring a bit addressing among instructions to be executed. A typical example is a bit field instruction. This instruction is used to perform an operation such as extraction, insertion, comparison or the like on data defined by a string of bits, i.e. a bit field, having an arbitrary length and a leading bit of an aribitary address (not only byte boundary address). Therefore, the bit address is required in order to execute the bit field instruction. However, since the minimum addressing unit specified by the addressing operand is the byte address as described above, the addressing operand of the bit field instruction also specifies the address of a memory in byte unit. For this reason, another operand called "bit offset operand" is required to represent offset information of the leading bit in the bit field to be processed from the byte address of the byte data containing the leading bit.
As in the above, the prior art addressing system makes use of two operands (namely, the base address operand and the bit offset operand) for showing the bit position, so that it becomes a double-addressing mode which takes time for address computation. Moreover, its encoder efficiency is low.